This invention relates to a railway car wheel and particularly to a railway car wheel suitable for use with railway cars which travel at high speeds or under heavy loads.
A railway car wheel comprises a hub which is connected to an axle, a rim which rests on a rail, and a web provided between them. The rim includes a flange and a tread. This invention relates to improvements in the rim and particularly the tread thereof.
In order to increase the efficiency of railway transport, railway cars are being operated under increasing loads and at higher speeds. As a result, the operating conditions of railway cars are becoming more severe.
Due to the more severe operating conditions, in recent years, rolling contact fatigue damage of the treads of railway car wheels caused by rolling contact with rails is becoming a problem. Since the frequency with which wheels are subjected to cyclic loads due to rolling contact is higher than for rails, the extent of fatigue damage and the wear rate tend to be greater for wheels than for rails. A higher level of the rolling contact fatigue resistance as well as the wear resistance than ever is required for the wheel.
Railway car wheels undergo two types of rolling contact fatigue damage. One type is referred to as shelling, and the other is referred to as spalling. Shelling refers to damage in which micro cracks initiate in the surface of a wheel due to cyclic rolling contact, and the micro cracks propagate to result in shelling. Shelling is correlated with the hardness of a material. Namely, resistance to shelling increases with hardness. Spalling refers to damage which initiates in an as-quenched martensite layer referred to as a white layer.
A white layer is formed at the time of rapid heating and rapid cooling of the surface of a wheel which is undergoing rolling. A white layer has a hard and brittle as-quenched martensite structure, so cracks initiate in an early stage and can easily lead to spalling. In order to prevent the initiation of cracks from a white layer, it is necessary to lower the hardness of the white layer. Since the hardness of a white layer, i.e., of as-quenched martensite is determined by the C content, lowering the C content is effective at increasing the resistance to spalling.
Conventionally, a high carbon steel has been used as a material for railway car wheels on account of its excellent wear resistance. Heat treatment of wheels made of high carbon steel has been carried out so as to give the treads of such wheels a pearlite structure. For example, Japanese Published Unexamined Patent Application 2000-345295 discloses a railway car wheel in which the tread and the flange have a pearlite structure with a prescribed C content and a prescribed hardness so as to attain a balance between wear resistance and shelling resistance. However, because this wheel has a pearlite structure, its C content is high, and its resistance to spalling is only of the same order as that of a conventional wheel.
Unfortunately, if the C content is simply lowered in order to improve spalling resistance, wear resistance and shelling resistance end up decreasing.
In this manner, with conventional railway car wheels, shelling resistance and spalling resistance could not be simultaneously increased, and their resistance to rolling contact fatigue ended up being poor. Thus, they could not cope with the recent trend toward higher speeds and higher loads of railway cars.